Tristetraprolin (TTP), a protein encoded by the ZFP36 gene, specifically induces the degradation of several inflammatory cytokines, oncogenes, and angiogenic factor mRNAs. Various authors have linked breast cancer (BC) aggressiveness to TTP expression deficiency. Furthermore, a single nucleotide polymorphism (SNP) in ZFP36 promoter region has been reported (rs251864) in which the AG or GG genotype is associated with worse prognosis compared to the AA genotype in Caucasian breast cancer patients in the USA. Here, in Argentina, we have found that the frequency of the allele G is approximately 50% each in a cohort of 54 BC patients. Interestingly, we have also found that the luminal BC cell lines MCF-7 and T47D have AA and GG genotypes at rs251864, respectively. Functionally, MCF-7 cells exhibit a rapid induction of ZFP36 RNA expression upon serum stimulation at 30 minutes and 1 hour. In contrast, T47D cells demonstrate a blunted response throughout the same time course. We then proceeded to clone out 640 bp including the polymorphic promoter region of each of these cell lines and to generate alternative reporter plasmids (pGL3-LUC). Luciferase assays using these constructs have also shown that the AA genotype exhibits serum-induced activation, while the GG genotype remains largely unresponsive. Additionally, we have identified a CpG island in the ZFP36 promoter, which co-localizes with the SNP and is a conserved regulatory element involved in serum-induced TTP expression. Therefore, we proceeded to treat T47D cells with the DNA methyltransferase (DNMT) inhibitor 5-Azacytidine for 48 hours before serum induction. Our results show that blocking DNMT activity sensitized T47D (GG) cells to serum treatment showing a similar response to MCF-7 (AA) cells. In summary, our data indicates that BC patients carrying the GG genotype at rs251864 might have a worse prognosis due to genetic and epigenetic events that are reducing ZFP36 transcription efficiency in tumor cells.